In order to prevent oxygen oxidation of various types of articles that are easily deteriorated or degraded by oxygen, such as foods, beverages, medicinal products, and cosmetics, and to store them for a long time, oxygen absorbers are used for removing oxygen inside the containers accommodating these articles.
As the oxygen absorber, an oxygen absorber including an iron powder as the base reactive compound is generally used because of its oxygen-absorbing ability, easiness in handling, and safety. The iron-based oxygen absorber is, however, responsive to a metal detector, and it is therefore difficult to use a metal detector for inspection of foreign matters. Furthermore, packaging bodies containing iron-based oxygen absorbers cannot be heated by a microwave oven because of a risk of ignition. Moreover, since the oxidation reaction of an iron powder needs water, the oxygen-absorbing effect is exhibited only when moisture-rich articles are stored.
Packaging containers and other containers having improved gas barrier properties and oxygen-absorbing properties have been developed by making the containers by a multilayer material including an oxygen-absorbing layer of an oxygen-absorbing resin composition composed of a thermoplastic resin and an iron-based oxygen absorber blended therein (see Patent Literature 1). Specifically, the oxygen-absorbing multilayer film includes an oxygen-absorbing layer disposed between the layers of a conventional gas barrier multilayer film composed of a heat sealing layer and a gas barrier layer, with an intermediate layer of a thermoplastic resin between the oxygen-absorbing layer and each of the layers of the conventional gas barrier multilayer film, depending on the case. The oxygen-absorbing layer is a thermoplastic resin layer dispersing an oxygen absorber therein. The oxygen-absorbing multilayer film is used as a film having the function of absorbing oxygen inside a container in addition to the function of preventing oxygen permeation from the outside and is produced by a known production method, such as extrusion lamination, coextrusion lamination, or dry lamination. However, this oxygen-absorbing multilayer film also similarly has disadvantages: The multilayer film is detected by a metal detector for foreign matter inspection of foods, etc., cannot be heated by a microwave oven, and shows the effect only on moisture-rich articles to be stored. The oxygen-absorbing multilayer film also has a disadvantage of insufficient internal visibility due to its opacity. An oxygen-absorbing multilayer film containing an oxygen absorber such as an iron powder has disadvantages: The multilayer film is detected by a metal detector for foreign matter inspection of foods, etc., has insufficient internal visibility due to its opacity, and reduces flavor when the contents are an alcoholic beverage due to generation of aldehyde by oxidation reaction of alcohol using iron as a catalyst.
If a container accommodating contents is sealed in the air, the air is taken and entrapped inside the container. Consequently, a certain amount of oxygen is taken inside the container and affects the contents in some way. Although the degree of the influence varies depending on the chemical properties of the contents, the flavor and color tone are gradually degraded after the sealing. The flavor and color tone are degraded by the influence of oxygen with an increase in storage period or distribution period after sealing, resulting in a reduction in commercial value. Thus, the existence of oxygen is not negligible, even if the amount is small.
For example, commercially available processed cooked rice is mainly stored as retort pouch cooked rice. In this storage, however, the rice is sterilized by heating under conditions different from common cooking conditions, leading to a disadvantage of degradation of the taste. Accordingly, sterilized cooked rice prepared by processing rice by a common cooking method in a sterile environment has been commercialized. Recently, a technique of wrapping such sterilized cooked rice with a multilayer body having an oxygen-absorbing function has been disclosed (see Patent Literature 7). Furthermore, a method for producing cooked rice by filling an oxygen-scavenging multilayer container with rice and water, performing substitution of oxygen inside the container, sealing the container, and then heating and cooking the rice has been disclosed (see Patent Literature 8). However, retort pouch cooked rice using an oxygen absorber such as an iron powder has disadvantages: The multilayer body is detected by a metal detector for foreign matter inspection of foods, etc.; if the multilayer body is used as the lid member, the iron powder adheres to the flange part when unsealed; the workability is low, and there is no visibility of the contents due to low transparency.
Technologies for filling metallic cans or glass bottles with alcoholic beverages, such as rice wine (Japanese sake), wine, or distilled spirit (shochu), and storing the alcoholic beverages are known. In these technologies, the metallic cans and glass bottles involve the problem of treatment of noncombustible waste and are required to be reduced in the weight of the packaging containers. In addition, metallic cans have the problem of dissolution of the metallic component into the contents and have been replaced with plastic containers such as gas barrier multilayer containers. When an alcoholic beverage is stored in a gas barrier multilayer body composed of usual gas barrier films, the operation in the air for filling a boron container with an alcoholic beverage necessarily causes contamination by the air. In order to prevent this contamination, although an inert gas, typically a nitrogen gas, is used for preventing the contamination by the air, the contamination by the air cannot be completely prevented, and such an operation in an actual production process increases the number of steps, resulting in a reduction in production efficiency. In other words, even if any gas replacement has been performed, a small amount of oxygen remaining in the packaging container or a small amount of oxygen dissolved in the alcoholic beverage inevitably causes a deterioration in the flavor of alcoholic beverages.
Furthermore, regarding drug solutions, as medical packaging containers for packaging and storing drug solutions in hermetically sealed conditions, for example, glass ampoules, vials, and prefilled syringes have been conventionally used. These glass containers, however, have problems: sodium ions and other components elute into the solution of the contents inside the container during storage; micro substances called flakes occur; when a light-shielding glass container colored with a metal is used, the contents are contaminated by the metal for coloring; and the container is easily broken by a shock such as falling. In addition to these problems, since glass containers have a relatively large specific gravity, medical packaging containers are disadvantageously heavy. Therefore, development of alternate materials has been demanded. Specifically, plastics lighter than glass, for example, polyester, polycarbonate, polypropylene, and cycloolefin polymers, have been investigated as glass alternatives. For example, a medical container made of a polyester-based resin material has been proposed (see Patent Literature 9).
Meanwhile, a plastic multilayer container including a gas barrier layer as an intermediate layer for providing a gas barrier property to the container has been investigated. Specifically, a prefilled syringe including the innermost layer and the outermost layer of a polyolefin-based resin and an intermediate layer of a resin composition having an excellent oxygen barrier property and thereby having an enhanced oxygen barrier property has been proposed (see Patent Literature 10). Furthermore, a multilayer container prepared by laminating a gas barrier layer on a resin layer has been investigated, where the gas barrier layer is made of, for example, a polyamide prepared from metaxylylenediamine and adipic acid (hereinafter, may be referred to as “nylon MXD6”), an ethylene-vinyl alcohol copolymer, polyacrylonitrile, poly(vinylidene chloride), aluminum foil, a carbon coat, or a vapor-deposited inorganic oxide. Furthermore, in recent years, nylon MXD6 provided with an oxygen-absorbing function by being mixed with a small amount of a transition metal compound has been proposed to be used as an oxygen barrier material constituting containers or packaging materials (see Patent Literature 11).
In the field of packaging medicinal products, foods, etc., press-through package (PTP) packaging bodies (may also referred to as blister package) are widely used for packaging medicinal agents such as tablets and capsules, granular foods, and other products. The PTP packaging is a form having a pocket portion for accommodating an article to be stored at the bottom formed by, for example, pressure molding or vacuum molding of a bottom member that is a plastic sheet of, e.g., a polyvinyl chloride resin or a polypropylene resin. The article to be stored is placed in the pocket portion, and the pocket portion is then sealed by laminating foil or a film made of a material that can be easily torn or broken by hand or be easily opened, such as aluminum foil, as the lid member. In the PTP packaging body, use of a transparent plastic sheet as the bottom member allows the article to be stored accommodated in the pocket to be directly observed with the naked eye before opening the packaging body. In addition, the article to be stored can be easily taken out by pressing the article from the pocket portion side with a finger to break the lid member.
However, if a PTP packaging body accommodating an article to be stored is sealed in the air, the air is taken and entrapped inside the packaging body. Consequently, a certain amount of oxygen is taken in the packaging body and affects the article to be stored in some way. Although the degree of the influence varies depending on the chemical properties of the article to be stored, the medicinal ingredients of medicinal agents and the flavor and color tone of foods are gradually degraded after the sealing. The quality is degraded by the influence of oxygen with an increase in storage period or distribution period after sealing. Thus, the existence of oxygen is not negligible, even if the amount is small.
In the aforementioned circumstances, an oxygen absorber including an organic material as a base reactive compound has been demanded. An oxygen absorber including ascorbic acid as the base compound is known as an oxygen absorber including an organic material as a base reactive compound (see Patent Literature 2).
Meanwhile, an oxygen-absorbing resin composition composed of a resin and a transition metal catalyst is known. For example, a resin composition composed of a polyamide as an oxidizable organic component, in particular, a xylylene group-containing polyamide, and a transition metal catalyst is known (see, Patent Literatures 3 and 4). Patent Literatures 3 and 4 exemplify packaging materials obtained by molding resin compositions.
An oxygen-absorbing resin composition composed of a resin having a carbon-carbon unsaturated bond and a transition metal catalyst is also known as an oxygen-absorbing resin composition not requiring moisture for absorbing oxygen (see Patent Literature 5).
Furthermore, a composition composed of a polymer containing a substituted cyclohexene functional group or a low molecular-weight substance bonded to the cyclohexene functional group and a transition metal is known as a composition for trapping oxygen (see Patent Literature 6).